Biaxially stretched polyester films are excellent in mechanical, thermal and electrical properties as well as chemical resistance and therefore are widely used in various fields. They are matched by no other polymer films especially for application as the base film for magnetic recording tapes. The quality of base films for magnetic tapes greatly influences the quality of the finished magnetic tapes and, therefore, quality standards required of base films for magnetic tapes are becoming higher and higher with progress in magnetic tape technology.
For instance, characteristics of a magnetic tape for video recording output, various S/N ratios, dropout, generation of envelop, etc. are related to flatness of the base film, whereas appearance of the wound-up tape, sliding property and wear resistance of a magnetic tape are closely related to the slipperiness of the base film. Thus, furnishing polyester films provided with both flatness and slipperiness is essential for production of magnetic tapes, especially those for video. Flatness and slipperiness are keenly desired characteristics of base films especially when a thinner magnetic layer or evaporated metal coating is employed in order to increase recording density.
The enhancement of the slipperiness of polyester films has hitherto been achieved by roughening the surface of the polyester film by externally incorporating or internally depositing minute particles. However, there is a limit in the effect of such methods in enhancement of flatness and they are not suitable for production of base films for high quality magnetic tapes for video recording in so far as the polyester films are produced under the conventional conditions. It has been considered that flatness and slipperiness are incompatible characteristics and that it is difficult to obtain a film satisfactory in the two characteristics in combination.
With respect to the polyester film represented by polyethylene terephthalate film, it has been known that biaxially stretched films excellent in slipperiness, which cannot be obtained by film-making at the ordinary stretching temperature, can be obtained by longitudinally stretching at a higher temperature. However, it is difficult to obtain a longitudinally stretched film with a uniform thickness by stretching an amorphous polyester film as prepared in single stage stretching at a higher temperature, since such a method generally increases irregularity in thickness, which eventually results in unevenness in thickness in the resulting biaxially stretched film. Multiple stage longitudinal stretching methods, which inhibit increase in thickness irregularity in the high temperature stretching are disclosed in Japanese Laying-Open Patent Publication No. 43772/73 and No. 122573/75. According to these high temperature longitudinal stretching processes, however, it is, at the highest, only up to the stretching temperature of 100.degree. C. that generation of thickness uniformity is inhibited. The increase in thickness irregularity is unavoidable in stretching at higher temperatures. That is to say, although it is expected that a biaxially stretched film with excellent flatness and slipperiness may possibly be obtained by longitudinally stretching the film at a higher temperature, it is difficult to stretch a film at such a high temperature in the conventional stretching because are thickness unevenness increases. In the case of magnetic tape, especially video magnetic tape, the requirement in the thickness uniformity is very high. The commercial value of a magnetic tape is reduced if the tape is unsatisfactory in the thickness uniformity no matter how excellent it is in its other properties.
We proposed a new process by which films can be longitudinally stretched at high temperatures with little increase in the thickness irregularity in the longitudinal stretching stage in a biaxial stretching process in Laying-Open Patent Publication No. 78729/83. However, the thus obtained biaxially stretched polyester film is not entirely satisfactory in the thickness uniformity even though it is flat and slippery. Therefore, a further improved process has been desired.
We continued an intensive study and as a result we found that the thickness uniformity after longitudinal stretching is further improved, if the index of birefringence of the film .DELTA.n at an early stage of longitudinal stretching is enhanced. Thus this invention has been achieved.
In the process proposed in Laying-Open Patent Publication No. 78729/83, it was a problem in the stretching procedure that films tend to stick to heated drawing rolls at the last stage of the longitudinal stretching, that is, at the high-temperature drawing stage. Two approaches are conceivable for solving this problem, that is, modification of the roll material and modification of the physical properties of the film itself. We have it possible to enhance the non-tackiness of the film by improvement of the stretching procedure. There is disadvantage in the other approach, that is, modification of the roll material. If adherence is reduced, durability and other desirable properties of the roll is sacrificed. Therefore, roll material shall preferably be left unchanged.